Gases that occur in nature or which are produced in industrial processes often contain carbon dioxide in small amounts. For example, atmospheric air generally contains about 350 parts per million (ppm) carbon dioxide. Because of certain process constraints or a particular end use that the gas is intended for, it may sometimes be desirable or necessary to remove the carbon dioxide from the gas. For example, air that is separated into various component products by cryogenic separation techniques (cryogenic air separation), such as cryogenic distillation or cryogenic adsorption, must be substantially free of both carbon dioxide and moisture, because these operations are carried out at temperatures below the freezing points of these compounds; consequently, if they are not removed they will freeze in and eventually clog the air separation process equipment.
Small amounts of carbon dioxide and moisture are removed from gas streams by various techniques, such as condensation, reversing heat exchange freezing and adsorption. A particularly preferred method is adsorption using an adsorbent which adsorbs carbon dioxide (and water vapor) more strongly than it adsorbs other components of the gas stream. For example, it is common to remove carbon dioxide from an air stream that is to be cryogenically separated, by passing the gas stream through a bed of zeolite 13X. U.S. Pat. No. 3,885,927, issued to Sherman et al. on May 27, 1975, discloses the use of type X zeolite containing at least 90 equivalent percent barium cations for the removal of carbon dioxide from gas streams containing not more than 1000 ppm carbon dioxide, at temperatures of -40 to 120.degree. F. U.S. Pat. No. 4,775,396, issued to Rastelli et al. on Oct. 4, 1988, discloses the adsorption of carbon dioxide from gas streams by pressure swing adsorption at temperatures of -50 to 100.degree. C., the adsorbent having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 2 to 100 and containing at least 20 equivalent percent of one or more cations selected from zinc, rare earth, hydrogen and ammonium cations and not more than 80 equivalent percent of alkali metal or alkaline earth metal cations.
Zeolite 13X efficiently removes small amounts of carbon dioxide (and water vapor) from air streams at low temperatures, i. e., temperatures of about 5.degree. C. or lower, because it more strongly adsorbs these components than it adsorbs nitrogen, oxygen or argon. However, the carbon dioxide adsorption capacity of zeolite 13X diminishes rapidly as the temperature of the gas being separated increases, and the separation process becomes infeasible at temperatures above about 20.degree. C. Since ambient 25 temperatures are often considerably above the preferred 5.degree. C. adsorption temperature, for example ambient temperatures of 40.degree. C. or higher are sometimes encountered, and since, because of the heat of adsorption and the heat of gas compression, there is a tendency for adsorption bed temperatures to increase considerably during the course of an adsorption process, it is usually necessary to cool the air fed to an adsorption-based air prepurification plant by means of external refrigeration to maintain the gas at temperatures below 20.degree. C. This reduces the overall efficiency of the air separation process, since energy must be consumed to provide the necessary refrigeration. U.S. Pat. No. 5,531,808, issued to Ojo et al. discloses the removal of carbon dioxide from gas streams by adsorption using as the adsorbent type X zeolite having as exchangeable cations one or more of various cations, including cations of Group 1A of the periodic table. U.S. Pat. No. 5,300,138, issued to Fischer et al. teaches that the performance of a rotary regeneratable dehumidification wheel is enhanced by using type X molecular sieves in which 20 to 60% of the sodium cations are replaced by potassium. The enhanced performance is attributed to a relatively lower sorption capacity for moisture on the partially potassium exchanged molecular sieve which allows the adsorbent to be regenerated at lower temperatures.
It would be desirable to find improved processes for removing carbon dioxide from gas streams using adsorbents with exceptionally high affinities for carbon dioxide. The present invention provides a carbon dioxide adsorption process which provides such an advantage.